Directional couplers are a very common element in RF and microwave systems. A directional coupler is a reciprocal, four-port circuit consisting of two pairs of ports in which (a) the ports of each pair are mutually isolated from one another, and (b) one pair of ports are matched. The ideal directional coupler takes power, which is incident at a first port, and transmits the power to two output ports with no transmitted power to a fourth port known as the isolated port. The quality of a coupler is measured by the insertion loss from an input port to an output port and by its directivity which is the ratios of power at the isolated port to the power at the coupler port.
Directional couplers operate on the principle of constructive and destructive interference of two waves. A signal at the input splits into two waves that arrive at the isolated port 180.degree. out of phase with one another and therefore cancel one another. At the direct and coupled output ports, these waves arrive in phase with one another and interfere constructively. A directional coupler may be used as a power level monitor, a local oscillator injection device, an attenuator, a power combiner/divider, or a device to produce a fixed relative phase angle between two signals.
At high frequencies, directional couplers are implemented by allowing two transmission lines to couple. Such directional couplers have been made by means of mechanical designs, stripline, microstrip and other types of transmission medium, relatively popular in a microwave field. See a text entitled "Microwave Semiconductors Circuit Design" by W. Allan Davis published by Van Nostrand Reinhold Company, 1984, Chapter 2 entitled "Passive Microwave Components".
The stripline broadside coupler is widely used and employs well established technology. The broadside coupler gets its name because the broadside of flat conductors effect the coupling. Typically such conductors are a quarter wave length long. The reason why the broadsides are used for coupling is that coupling from the edges is insufficient to form heavily coupled devices, such as 3 dB couplers. Because the stripline coupler has two ground planes and a homogeneous dielectric, TEM propagation occurs and the even and odd phase velocities are identical which gives good bandwidth, directivity, and VSWR. It is understood that directivity is the measure of how much signal is present at the isolation port.
Another type of coupler in widespread use is the microstrip coupler. Microstrip has only one ground plane with the conductor supported by a layer of dielectric and therefore does not truly support TEM propagation. This type of coupler has two major problems. The first problem is the fact that it is very hard to manufacture a 3 dB coupler because the dimensional separation is very tight and therefore extremely critical. Prior art approaches have attempted to solve that problem by using interdigital techniques, as by interdigitating the coupler. The lack of true TEM propagation causes another problem. Because the even mode travels in the dielectric and the odd mode (the coupling fields between the conductors) travels in the air and dielectric the odd mode travels faster, reducing the directivity and bandwidth of the coupler. This is a major problem with microstrip.
It is desirable to use microstrip for a coupler as microstrip is used in a majority of microwave integrated circuits (MICs) and monolithic microwave integrated circuits (MMICs).